1. Field of the Art
The present invention relates, in a first aspect, to a process for calculating distances between wireless nodes, based on the calculation of the round trip time, or RTT, between the sending and the reception of respective data packets or frames, and more particularly to a process which comprises calculating said round trip time by means of providing and using software layers in a node to be positioned.
A second aspect of the invention relates to a system for calculating distances between wireless nodes which comprises a wireless node to be positioned configured to implement the process proposed by the first aspect.
The invention has, among other applications, those referred to mobile communications, geolocation and telematic engineering.
2. Prior State of the Art
The calculation of the time of arrival of packets transmitted or received in a node is an essential process in the positioning by means of multilateration systems. There are multiple approaches to this problem, although the proposed processes can be classified into two large groups: hardware and software solutions. The first of the groups comprises those solutions in which, to achieve measurements of times of arrival in wireless network nodes, the hardware of said nodes is modified. It is thus achieved that the measurements are taken at the precise instant in which the frames are received or transmitted by the network node. The main drawback of this type of approach for calculating the times of arrival is that, since it requires hardware modifications, it is not applicable to conventional network devices and therefore cannot be directly applied to already deployed communications systems. An example of this can found in the proposal for WLAN networks published in [1], where a process for calibrating the delay in transmitter and receiver nodes by means of RTS/CTS is performed. [2] proposes obtaining the timestamps corresponding to the receptions and transmissions of packets by means of capturing a waveform segment of the transmitted signal to filter it and compare it with a known sequence. Other proposals in this sense have chosen to suggest changes in the “firmware” of the network interfaces of the nodes to be positioned, thus reducing the hardware modifications to a minimum. Some of the most significant contributions in this sense for WLAN networks are those which can be found in [3-5].
The second of the groups of solutions for calculating the times of arrival is based on modifying the software controlling the node, using the existing hardware capabilities to calculate the timestamps. Most of these solutions are based on time-stamping the transmission of a packet towards a reference node and the subsequent reception of said packet (or another response) in the source node (RTT). To that end, they alter the link level of the protocol stack of the device, although each of them chooses different strategies.
An approach of said software solutions consists of using a network interface capable of providing time measurements made by the hardware. The advantage of this approach is that they are usually solutions using standard hardware, directly applicable to any already deployed system. However, the timestamps performed with this type of process usually do not have an acceptable accuracy for most location applications. For example, in the case of WLAN, the characteristics of the IEEE 802.11 standard [10] would make said resolution be 1 microsecond, which corresponds to 300 meters of error in terms of distance. Furthermore, IEEE 802.11 only contemplates performing timestamps in the reception of the frames and not in the transmission. A representative process of this option is the one presented in [6], which uses the node to be positioned and a monitoring node to capture RTTs by means of using the “tcpdump” command. To reduce the error derived from the system of measurements, said system chooses to take a large number of samples and refine them by means of a statistical filtering process. The authors thus achieve errors in distance of the order of 8 meters, a value which can be considered to be poor for multiple services and environments. In 2008, the same authors proposed a location system called Goodtry [7], which uses a technique similar to the one set forth in [6] but to which several new characteristics which are theoretically capable of improving the accuracy obtained are added.
It is necessary to offer an alternative software solution to those included in the approach described in the previous paragraph which allows obtaining higher accuracy in the calculation of RTTs and which, in contrast to the mentioned software solutions, does not use the network interface to obtain the time measurements, i.e., it is transparent to the network interface used and can be implemented in the actual node to be positioned, without needing to use a monitoring node.